Not only did a chippie do that, but a chippie with 40 years experience - as he repeatedly pointed out to me, in-between moaning about the doorway, and telling me how much he was looking forward to retirement. He had to make up the liner from a single plank of wood because the standard door liners were smaller than the doorway width. The plank, as you say, clearly wasn't wide enough, but since the builders weren't brought in to do anything after the fitting of the door liner, what do they care? I have the standard door liner kit that he was originally intending to use going spare now.
The huge gaps round the liner are because the doorway was widened - badly - by his builder colleagues. And the reason the liner had to be shaved off at the corner was because the lintel above is getting dangerously close to the edge of widened door opening. It's unfortunate because he had to reduce the standard size from 30 inches to 29 3/4 inches because of that lintel problem, so when the door does eventually go in it'll need a fair bit of 'trimming' to fit. What's more, the wall slopes, so on the other side, there is an overlap of liner over plaster.
The doorway was originally very narrow, and I wanted to widen it to get larger furniture in there. When the original door liner was removed they found that the original builders had put bricks not only above the lintel, but below the lintel. Go figure. So after removing the bricks below, and installing the liner, there was still a gap from liner to lintel.
I won't be using the building firm in question again. They did a generally shonky job, and while they broadly met the spec, their main concern was to do everything at breakneck speed, while charging through the nose (the inflated charges made me expect quality, more fool me). In fact, this is part of the reason that I'm getting into DIY, because I have diminishing faith in the willingness of builders to do a careful - and well finished - job. The problem seems to be finding someone who can make a decent job of it. The majority of the builders that I've had in to quote for this job never got back to me with a quote, even after I chased. Which says to me that any or all of the following are true:
1. the recession in the building trade round here is a myth
2. they didn't fancy the job, so builders are still being picky over the jobs they do in my area (see 1.)
Rant over.